The spread of Soybean Cyst Nematode (SCN) across Minnesota has entered a new phase. In 2004, 2005 and 2006 SCN was found in soil samples from Wilkin and Clay counties in northwestern Minnesota and in nearby Richland and Cass counties in North Dakota. First found in Faribault County in 1978, SCN has taken almost thirty years for SCN to spread throughout 58 counties in the southern half of the state (Fig. 1). Because of annual flooding of the Red River and its tributaries, local edaphic factors and weather conditions, and unique management and cropping practices, SCN will spread quickly throughout the remaining uninfested counties in the Red River Valley.

Fig. 1. Spread of soybean cyst nematode (SCN) in Minnesota. SCN was first found in Minnesota in 1978 in Faribault county (arrow). In 2006 the presence of SCN was confirmed in Clay county.
(Map courtesy Dr. Senyu Chen, U of MN)

Spring flooding by the Red River and its tributaries will play a major role in dispersal of SCN in the Red River Valley. The infested fields in Clay and Wilkin counties are located near tributaries of the Red River. Spring floods are a frequent, almost annual occurrence along the Red River which flows northward into areas that are currently thought to be uninfested. In the past floodwaters have extended as much as a mile from the streambed occupied by the Red River during normal flows. The potential for rapid, widespread movement of SCN is very different from the relatively slow usually short distance field to field movement that occurred in southern Minnesota.

Soil Temperature is a poorly understood influence on SCN populations that will determine the pace of SCN population increase and final nematode numbers. Parasites of nematodes, including predaceous nematodes, and parasitic fungi and bacteria, are an important influence on SCN populations. In warmer states such as Missouri these parasites act as biocontrol agents limiting SCN numbers. A significant portion of their activity takes place during fall and into the winter if soils are not frozen. It is likely that the activity of potentially beneficial parasites may be greatly reduced in the Red River Valley when compared to areas where above-freezing soil temperatures occur for much longer periods of time.

High soil pH has been associated with rapid population increases and higher final populations of SCN. Research conducted in field surveys, plot experiments, and greenhouse studies in southern Minnesota indicates that SCN population densities are positively correlated with soil pH. Research conducted in Wisconsin by Dr. Craig Grau suggests that SCN populations increase more rapidly when susceptible soybean varieties are grown on soils with pH 7.0 or greater than when the same varieties are grown on soils of pH 5.9–6.5.  The high pH soils present in the Red River valley may support a more rapid increase in SCN numbers and greater final populations than that experienced in southern Minnesota.

Cropping and management practices are an important influence on SCN populations. A critical factor in the spread of SCN is movement of soil from infested to uninfested fields. Movement of residual soil on tillage and harvest equipment has probably the most important means of spreading SCN in southern Minnesota. Cropping practices that are unique to the Red River Valley pose added risk for accidental spread of infested soil. Potato or sugar beet production on widely dispersed fields has the potential to move infested soil over large areas. Sugar beet production practices such as planting, topping, and harvesting necessitate the movement of equipment from field to field throughout the Red River Valley. In addition, the practice of mingling “tare” soil accumulated from many different fields, and then returning this soil to different fields increases the risk of moving SCN from infested to uninfested fields.

Continuous planting of susceptible soybean varieties or other susceptible crops such as dry beans will accelerate the buildup of SCN populations. The very limited number of resistant soybean varieties currently available in early maturity groups (MG 0 and I) limits options for managing SCN in the Red River Valley. Susceptible crops such as dry beans are ideal hosts for SCN. Research conducted by Dr. Berlin Nelson at North Dakota State University indicates that SCN populations can increase as rapidly on some kidney bean varieties as they can on susceptible soybeans. Although SCN does not reproduce as well on other dry bean cultivars, they will support significant populations of SCN, negating any benefit from rotating to dry beans as an alternate crop.

Short term and long term implications of SCN infestation confront not only soybean producers but also growers of other crops in the Red River Valley. The most serious consequence of an SCN infestation is, of course, the reduced yields that occur where SCN is present. This effect is compounded by the interaction of SCN with Fusarium root rots, Sudden Death Syndrome, and Brown Stem Rot resulting increased severity of these diseases. The presence of SCN in fields in the Red River Valley will have special implications for growers involved in the export of seed potato or other horticultural crops. Currently some countries restrict imports of these items from areas where SCN is known to be present. It is left up to the producer to certify that their fields are free of SCN, a difficult and costly process. In addition the presence of SCN in fields will dictate rotation sequences consisting of non-host crops to avoid buildup of damaging SCN. infestations.

Reducing losses to SCN requires that the spread of SCN into uninfested areas be slowed and that SCN numbers be limited as much as possible. In very general terms the following steps are the basis of SCN management.

• Identification of infested fields is an essential first step.
• Control of movement from infested to uninfested fields.
• Crop rotation with non-host crops to limit SCN numbers.
• Planting SCN resistant varieties to limit SCN numbers and maintain yields.

A companion E-News article Managing soybean cyst nematode
in the Red River Valley discusses SCN management and
provides information about diagnostic resources that are available to soybean growers in the Red River Valley, Minnesota, and North Dakota.

Soybean has become a dependable, profitable mainstay of the agricultural economy in the Red River Valley and Northwestern Minnesota. Since 1995 soybean acreage in this area has increased almost five-fold yielding more than 40 million bushels of soybeans annually. The introduction of SCN presents a serious challenge to soybean productivity but it is a challenge that can be dealt with.
           

printer-friendly pdf